Electron beam lithography is widely used in the manufacture of semiconductor devices on silicon and other semiconductor wafers. The electron beam is capable, because of its small wavelength, of exposing patterns in a suitable electron resist which are smaller than those which can be made with visible or ultra-violet light. The electron beam is also easily steered by a time varying deflection voltage or current to expose the pattern.
It has two main drawbacks compared with light for lithography. Firstly the electron beam forming components, usually stacked above each other, with the gun at the top and the exposure surface at the bottom, and with lenses, apertures and other components between, the whole being therefore known as the `electron beam column`, or simply the `column`, must operate on the beam in high vacuum, whereas light can be used in air. Secondly electron optical lenses have very large aberrations compared with light lenses, which have made it so far impractical to use electron beam projection systems in which a patterned mask is placed in the path of the beam, thus causing a negative of this pattern to be projected on the resist surface to be exposed. Projection systems using light on the other hand are commonly used in lithography.
In high throughput lithography therefore it is common to combine the virtues of the electron beam with those of light. First the electron beam is used to produce a mask or reticle which bears the pattern, by exposing electron resist coated on to the mask surface. This is done by serial movement of a small focused electron beam to expose the pattern pixel by pixel. Secondly a light based system is used to expose in photoresist coated on the surface of the semiconductor wafer an image of the previously exposed and developed mask by allowing the light to pass through that pattern. Thus the pattern for a given layer on the semiconductor wafer can be exposed in one light exposure taking a fraction of a second, as compared with the same exposure of the original mask pattern which can take up to several hours. Usually a refinement of this procedure is used in which a stepper is used to expose the wafer by exposing one part of the complete wafer pattern, then stepping to the next and so on over the wafer surface.
Sometimes the mask or reticle pattern will be exposed at four times, or some other multiple of, the required final scale. The light exposure will then also include a reduction, or demagnification of the mask pattern by the scaling factor, so that the final pattern exposed on the wafer surface is the required size.
The exposed pattern in wafer borne photoresist will then be developed and used as a mask in itself for one step in the processing, for example ion implantation or metal vapour coating, of the semiconductor wafer towards the final target of manufacturing the microcircuit or chip. The complete wafer process will usually require about 20 such processing steps, each of which requires a different exposed and developed pattern. So a given wafer will require for full processing typically twenty masks or reticles.
In exposing electron resist on the surface of the mask or reticle the electron beam is deflected over the surface to produce serial, pixel by pixel exposure. This deflection can be produced either by so called `vector deflection` methods, in which the beam is moved rapidly to an exposure region, a small region is exposed, and the beam is then moved directly on to the next exposure region, or by `raster scan` methods. In raster exposure the beam is scanned over the complete surface in a raster pattern and is turned off or `blanked` where exposure is not required.
The most commonly used method for high throughput mask production is the raster scan method. Since this requires that the beam be turned on for every pixel which is exposed, and off for every unexposed pixel, and since exposure patterns are unpredictable, the raster scan system must be capable of turning the beam on from off, or off from on, for every pixel. This results, for modern, high throughput raster scan systems, in a requirement that the beam blanker can run at repetition rates of 320 MHz and higher.
Fast beam blankers required for high blanking repetition rates such as this are almost always of the deflection blanking type. In this type the electron beam passes between one or multiple deflection plate pairs each of which is usually disposed symmetrically about the column axis and is blanked by deflecting it off the axis. An aperture with its central hole disposed on the column axis will stop, or blank, the beam when this deflection is greater than the radius of the hole.
Each plate pair consists of two conductors. Each conductor in the pair will have a plane inner surface, or a singly or multiply curved or other compound inner surface. A potential difference, known hereinafter also as a voltage, will be applied between the two plates so that an electric field exists in the space between the plates, through which the electron beam proceeds. An electron passing through this field will be deflected by the electric field. That deflection will carry the electron away from the column axis and when the deflection is large enough the beam will be stopped on the outer regions of the aperture.
The velocity of an electron which passes through this space between the plates is finite and dependent upon the voltage through which it has been accelerated before it emerges from the electron gun. If the time taken by the electron to pass through the space between the plates in which the deflection field exists is comparable with, rather than very short compared with, the time for which the beam is blanked off during pattern exposure, then operational problems can arise. The focused electron beam at the mask surface can thereby move during blanking. Any movement during blanking affects the area of resist exposed during the pixel exposure and can therefore cause faults in the exposed pattern.
This problem has been addressed partially in the prior art, especially by the "horseshoe blanker". In this design two pairs of deflection plates are used, together with a blanking aperture disposed at the center of the whole structure, equidistant from each plate pair and on the beam axis. The center of the blanking aperture is made electron optically conjugate with the intersection of the mask surface and the column axis, by a succeeding electron lens. The upper plate pair and the lower plate pair are both portions, also, of a parallel strip transmission line which proceeds horizontally past the column axis, then bends around in an arc and returns horizontally back past the column axis, parallel with its top section, in other words in a horseshoe shape. The blanking voltage pulse is applied to the beginning (top) of the horseshoe and proceeds along the transmission line to the end (bottom) of the horseshoe thus passing the column axis twice. The propagation time of the blanking voltage pulse along this transmission line from the top column axis intersection to the bottom column axis intersection is designed to equal the time taken by an electron to travel by its more direct route, close to the column axis, from the top intersection to the bottom intersection.
In the steady state, with a DC voltage applied between each plate pair, the top pair deflects the beam off the axis by a given angle and the bottom pair deflects the beam further in the same direction by an equal angle. Thus, in the steady state, with a deflection voltage applied, the focused beam at the mask surface remains in the same place as with no voltage applied, since the angle of beam emergence from the bottom pair is such that it appears to originate from the center of the blanking aperture at all values of the deflection field and the center of the blanking aperture is conjugate with the intersection of the mask surface and the column axis.
The horseshoe blanker solves the problem only partially however, since each electron travels at a finite speed and takes a finite time to pass through each of the two plate pairs. The result is that an electron can suffer different amounts of deflection which depend upon where the electron was within the plate pair when the blanking voltage pulse started and stopped. This means that the deflection suffered in the second plate pair may be greater or less than the deflection it suffered in the first plate pair. This, in turn means that the electron will not usually appear to emerge from the center of the blanking aperture, its point of origin will not be conjugate with the mask surface/column axis intersection and it will not therefore go to the correct place on the mask surface. Therefore the focused spot on the mask surface will move during blanking and the exposed pattern will be spoiled.
Further limitations of the horseshoe blanker are as follows.
At 320 MHz blanking repetition rate the transmission line of the horseshoe blanker is no longer immune to parasitic capacitance and inductance especially those which occur between the top and bottom arms of the horseshoe. Such parasitics cause distortion of the waveform as it travels along the transmission line. If the deflection voltage waveform at the bottom plate pair is different from that applied at the top plate pair this also causes movement during blanking.
At 320 MHz blanking repetition rate, reflections and losses at the vacuum wall located plug and socket transitions between electronic blanking driver and the transmission line cause distortions and differences between the voltage waveforms applied at the top and bottom plate pairs.
At 320 MHz the mechanical adjustment of delay which is required to match the delays of the electron and the blanking voltage pulse are unacceptably coarse.
There is a need for a new embodiment of the dual plate pair deflection blanking system to eliminate the movement of the focused beam at the mask surface during blanking due to the finite passage time of the electron past each of the plate pairs.
There is a need to make negligible any differences between the voltage waveforms applied at the top and bottom plate pairs.
There is a need to provide means of fine delay adjustment which allows for blanking deflection repetition rates of 320 MHz and above.